1. Field of the Invention
This invention relates to methods and apparatus for producing a hydrogen containing gas from a hydrocarbon feedstock.
2. Description of the Prior Art
In the prior art, producing a hydrogen containing gas, such as hydrogen, from a hydrocarbon feedstock is typically accomplished by passing the feedstock (and steam if the conversion process is steam reforming) through catalyst filled tubes disposed within a furnace. Fuel and air are burned within the furnace to provide heat for the catalytic reaction taking place within the tubes. In order to improve the efficiency of such apparatus some efforts have been directed to improving the uniformity of heat distribution to the tubes within the furnace while minimizing the amount of energy used to produce each unit of hydrogen containing gas. For example, in commonly owned U.S. Pat. No. 4,098,587 to R. A. Sederquist et al the reaction tubes are clustered closely together in a furnace, with baffles and sleeves surrounding each tube to improve heat transfer from the combusting gas in the furnace into the catalyst beds. Each catalyst bed is annular; and a portion of the heat in the product gases leaving the bed is returned to the bed to further the reaction process by flowing these product gases through a narrow annular chamber along the inside wall of the annular catalyst bed. The example given in column 7 of the Sederquist et al patent indicates that an overall reactor thermal efficiency of 90% was achieved with the apparatus described therein. Other commonly owned patents of a somewhat similar nature are 4,071,330; 4,098,588; and 4,098,589.
One drawback of the approaches taken in all of the foregoing patents is that the heat for the conversion process is still provided indirectly by means of heat transfer through reactor walls. Also, a considerable amount of heat energy leaves the furnace with the furnace exhaust gases. Although some of this heat can be recovered and used for other purposes, such as producing steam, it would be more beneficial if this heat energy could be used in the conversion process.
Another process and apparatus for the catalytic conversion of hydrocarbons by steam is shown and described in a paper titled "Conversion Catalytique et Cyclique Des Hydrocarbures Liquides et Gazeux" published by Societe Onia-Gegi. That system comprises a first vessel including a first heat exchange chamber, followed by a second vessel containing a catalyst bed, followed by a third vessel including a second heat exchange chamber. In operation, steam is introduced into the first vessel and is preheated as it passes through hot checkerbricks disposed within the chamber. Downstream of the checkerbricks the preheated steam is mixed with a hydrocarbon feedstock and the mixture passes into the second vessel containing a heated catalyst bed by means of a conduit interconnecting the two vessels. Conversion takes place as the mixture passes through the heated catalyst bed. Hot conversion products leave the second vessel and enter the third vessel, whereupon the hot conversion products give up heat to checkerbricks which are disposed therein. The conversion products may then be stored or used directly.
When the temperatures in the first heat exchange chamber and in the catalyst bed are too low to convert the feedstock, the apparatus is switched to a regeneration cycle. In the regeneration cycle air is introduced into the third vessel and is preheated as it passes through the checkerbricks disposed therein which were heated during the conversion cycle. Downstream of the checkerbricks a fuel, such as oil, is mixed with the preheated air and combusts. In order to keep combustion temperatures within acceptable limits, air in excess of that required for stoichiometric combustion is used. The hot combustion products are directed into the second vessel and pass through the catalyst bed, therein heating the same. This is the heat which is used during the conversion cycle. Because of the excess air, the catalyst bed is oxidized, although this is not desirable. (During the conversion mode of the cycle the oxidized catalyst is reduced back to the metal; this requires use of some of the hydrogen being manufactured, and has a negative impact on efficiency).
After passing through the catalyst bed the combustion products are directed into the first vessel and give up additional heat to the checkerbricks disposed therein. This is the heat which is used to preheat the steam during the conversion cycle.
Commonly owned U.S. Pat. No. 3,531,263 describes an integrated reformer unit comprised of a can-type structure which houses the reaction components of a system for converting hydrocarbon feedstocks to hydrogen. This compact apparatus, in one embodiment, comprises a center tube containing a volume of reform catalyst, followed immediately by a region of heat transfer packing material, followed by a volume of shift conversion catalyst. Surrounding the tube over its entire length is an annular passage. Air is introduced into the end of the annular passage adjacent the shift catalyst volume of the center tube. It is mixed with fuel approximately adjacent the interface between the heat transfer packing material and the reform catalyst. The fuel and air burn and travel further downstream around the outside of that portion of the center tube carrying the reform catalyst. Simultaneously a mixture of a hydrocarbon feedstock and water enter the center tube at the reform catalyst end. Steam reforming takes place within the catalyst bed with the heat being provided by the hot combustion products flowing countercurrent in the annulus around the outside of the tube. As the reform products leave the catalyst bed they give up heat to the heat transfer packing material in the next following region. This heat is used to preheat the air flowing around the outside of this heat transfer region before the air is mixed with the fuel and burned. The cooled products from the packing material region then pass through the shift conversion catalyst volume whereupon carbon monoxide present therein is converted to additional hydrogen and carbon dioxide. This reaction is exothermic, and the heat produced thereby preheats the air flowing around the outside of the tube.
While the foregoing apparatus is compact, and careful attention has been given to the overall heat balance and heat requirements of the hydrogen generating reaction, most heat transfer is still indirect and a significant amount of the heat energy generated within the apparatus, leaves the apparatus with the combustion exhaust and the reform products.